We can get a sense of how successful and lasting this edict will be with a simple mental experiment: replace "Bitcoin" with "US Dollar." The edict thus becomes: "the following US Dollar activities are illegal in Thailand: buying US Dollars; selling US Dollars; buying any goods or services in exchange for US Dollars; selling any goods or services for US Dollars; sending US Dollars to anyone located outside of Thailand; receiving US Dollars from anyone located outside of Thailand." How successful could that edict be, and how long could it last, realistically?
Foreign exchange controls are normally difficult to implement and enforce. Control of Bitcoin -- a decentralized, optionally anonymous, virtual commodity -- should be even more difficult.
I suspect a lot of Bitcoin trading volume in Thailand will simply go underground.